FIG. 1 is a cross-sectional view illustrating a conventional MOS transistor. As seen in FIG. 1, a conventional MOS transistor has a gate electrode 14, a source region 16a, and a drain region 16b. The gate electrode 14 is formed on a semiconductor substrate 10 wherein a gate oxide film 12 is interposed between the gate electrode 14 and the semiconductor substrate 10. The source and the drain regions 16a and 16b are formed on the semiconductor substrate 10 and the gate electrode 14 is disposed between the source and the drain regions 16a and 16b. The source region 16a supplies carriers (electrons or holes), and the drain region 16b extracts the carriers. The gate electrode 14 serves to form a channel in which the source region 16a is electrically connected to the drain region 16b. As is further illustrated in FIG. 1, an insulating spacer 18 may also be formed on the sidewall of the gate electrode 14.
As semiconductor devices become more highly integrated and the length of the gate electrode is reduced, the channel length (distance from source to drain) of the MOS transistor is, typically, also reduced. In a MOS transistor, the distribution of the electric field and the electrical potential in the channel region should be controlled by the voltage applied to the gate electrode. However, as the channel length of the MOS transistor is reduced, the channel region may be affected not only by the voltage applied to the gate electrode but also by the charge and/or the distribution of the electric field and/or electrical potential in the depletion layers between the source and the drain regions. Such affects may result in detrimental punch through or leakage currents.
Punch through may result from the depletion layer in the source region with contacting the depletion layer in the drain region. Punch through is illustrated in FIGS. 2 and 3. FIG. 2 is a schematic cross-sectional view showing the drain depletion layer of a conventional MOS transistor when the drain voltage is approximately 3V. FIG. 3 is a schematic cross-sectional view showing the drain depletion layer of a conventional MOS transistor when the drain voltage is approximately 7V.
As shown in FIGS. 2 and 3, the drain depletion layer approaches the source region because the drain depletion layer extends in proportion to the increase in the drain voltage. Thus, when the channel length is shortened, the drain depletion layer may be connected to the source depletion layer. In such a case, current can flow between the source and the drain regions S and D without the formation of the channel because the drain electric field affects the source region S so as to reduce the diffusion potential near the source region S. Such a phenomenon is referred to as a “punch through.” When punch through occurs, the drain current rapidly increases without the need for saturation in a saturation region of the transistor.
In addition, an electric field is generated between the bulk region of the substrate and the drain region D when a voltage is applied to the drain region of the MOS transistor. If the drain region D is highly doped, leakage current may flow from the drain region D to the bulk region of the substrate because the strength of the electric field between the bulk region and the drain region D increases. Thus, failures may occur in a semiconductor device, such as a memory device, in which charge should be preserved when the charge is leaked as a result of the leakage current. Thus, conventionally, the drain region D is not highly doped. However, without a highly doped drain, the contact resistance between the drain region D and the conductive material contacting the drain region D may increase. Also, the contact resistance may increase with the reduction of contact area between the drain region D and the conductive material that may result from decreasing the area of the MOS transistor.
To overcome the above-mentioned problems, U.S. Pat. No. 5,981,345 discloses a method for isolating the source region from the drain region and using a silicon film or a silicon-germanium (Si—Ge) film as the channel region. However, the formations of the source and the drain regions may be difficult to control because the source and the drain regions are formed by diffusing the impurities doped in a conductive film for the source and the drain regions toward the channel region.